This will be Gmail not recognizing the newest version of Thunderbird as "Legitimate" I suggest you verify them as legitimate. Hopefully Google will get the message.

I'm getting this issue everyday now, multiple times a day sometimes, even updating my passwords doesn't help, Gmail will still prevent login and mark it as suspicious even with the correct password, I have to login via webmail to get in. I've marked it as me each time but gmail still keeps bringing this up everyday.

This new Thunderbird is broken, I'll roll back and avoid updates.